Comparative morphology of Pliocene, Quaternary and Recent shells of Ocenebra erinaceus (Linnaeus, 1758) and O. brevirobusta Houart, 2000 (Mollusca, Muricidae, Ocenebrinae): reflections on the intra- and interspecific variations

The morphological variations of Pliocene, Quaternary and Recent shells of Ocenebra erinaceus (Linnaeus, 1758) and O. brevirobusta Houart, 2000 are analyzed. The study of these oyster drillers is based on various methods (cladistics, biometry, Procrustes and Fourier analyses) and concerns populations from 40 localities of the Atlantic (from Normandy to Morocco) and Mediterranean shorelines. The results indicate a morphological distribution of North/South orientation, the more derived southern forms being closely related. In spite of a strong variability, a beginning of differentiation, probably dating from the Plio-Pleistocene, is observed between three groups: 1) Northern Atlantic Ocean (O. erinaceus); 2) Mediterranean Sea (O. erinaceus); and 3) Moroccan Atlantic (O. brevirobusta). O. brevirobusta is temporarily regarded as a geographic subspecies of O. erinaceus, because of its uninterrupted marginal area, and its P2 cord more atrophied than in the Italian late Pliocene O. erinaceus. Within these groups, the multivariate analysis of variance distinguishes local biogeographic entities. In addition, two unexpected results deserve to be stressed: 1) the population from Étang de Thau, systematically associated to those of the Northern Atlantic Ocean, is located in a zone of oyster importation from the Oléron island; and 2) the population from Algarve (Portugal), closer to those of the Mediterranean Sea, lives in an Atlantic area undergoing the influence of Mediterranean waters. Comparisons between O. erinaceus and Ocinebrellus inornatus (Récluz, 1851), an Asian species introduced on the French Atlantic coast, show more similarities with the Northern Atlantic O. erinaceus, than with Mediterranean or Moroccan Atlantic O. erinaceus.